The method disclosed herein relates to printing systems that generate images onto continuous web substrates. In particular, the disclosed embodiments relate to printhead alignment in such systems.
Printers provide fast, reliable, and automatic reproduction of images. The word “printer” as used herein encompasses any apparatus, such as a digital copier, book marking machine, facsimile machine, multi-function machine, etc. which performs a print outputting function for any purpose. Printing features that may be implemented in printers include the ability to do either full color or black and white printing, and printing onto one (simplex) or both sides of the image substrate (duplex).
Some printers, especially those designed for very high speed or high volume printing, produce images on a continuous web print substrate. In these printers, the image substrate material is typically supplied from large, heavy rolls of paper upon which an image is printed instead of feeding pre-cut sheets from a bin. The paper mill rolls can typically be provided at a lower cost per printed page than pre-cut sheets. Each such roll provides a very large (very long) supply of paper printing substrate in a defined width. Fan-fold or computer form web substrates may be used in some printers having feeders that engage sprocket holes in the edges of the substrate.
Typically, with web roll feeding, the web is fed off the roll past one or more printhead assemblies that eject ink onto the web, and then through one or more stations that fix the image to the web. A printhead is a structure including a set of ejectors arranged in at least one linear array of ejectors, for placing marks on media according to digital data applied thereto. Printheads may be used with different kinds of ink-jet technologies such as liquid ink jet, phase-change ink, systems that eject solid particles onto the media, etc.
Thereafter, the web may be cut in a chopper and/or slitter to form copy sheets. Alternatively, the printed web output can be rewound onto an output roll (uncut) for further processing offline. In addition to cost advantages, web printers can also have advantages in feeding reliability, i.e., lower misfeed and jam rates within the printer as compared to high speed feeding of precut sheets through a printing apparatus.
A further advantage is that web feeding from large rolls requires less downtime for paper loading. For example, a system printing onto web paper supplied from a 5 foot diameter supply roll is typically able to print continuously for an entire shift without requiring any operator action. Printers using sheets may require an operator to re-load cut sheet feeders 2 to 3 times per hour. Continuous web printing also provides greater productivity for the same printer processing speed and corresponding paper or process path velocity through the printer, since web printing does not require pitch space skips between images as is required between each sheet for cut sheet printing.
To achieve the high speeds desired in continuous web printing and to cover the width of the web as required in production printing, multiple printheads are used. As the printer operates, the printheads expand and contract in response to changing thermal conditions. Thus, the width covered by a particular printhead (the “extent” of the printhead) varies depending on the operating temperature. Likewise, the rollers used to define the process path expand and contract in response to temperature changes. The expansion and contraction of the rollers affects the alignment of the process path. “Alignment” as used herein, unless otherwise expressly qualified, is defined as the location of the printhead along the width of the process path immediately adjacent to the printhead (cross-process location), and the orientation of the cross-process axis of the printhead with respect to an axis perpendicular to the edge of the process path. Thus, the web, which is designed to move perpendicularly past each of the printheads, may move past a printhead at a skewed angle when the printhead is misaligned. Additionally, the cross-process extent of the printhead may not be positioned properly with respect to the other printheads.
Misalignment resulting from movement of the printheads and the rollers is exacerbated by the positioning of printheads for different colors at different locations along the process path. Specifically, printers that generate color copies may include one or more printheads for each color of ink used in the printer. Each of the printheads associated with the different colors is positioned at a location along the process path that may be separated from other printheads by one or more roller pairs. Each roller pair produces a unique alignment of the media with respect to the process path. Accordingly, changes in the printheads and rollers may cause the printheads to be misaligned with the web as it moves along the process path.
Alignment of printheads in a printer is typically accomplished by bringing the printer up to its operational speed and printing a series of marks on the continuous web. The positions of the printed marks are detected by a scanner and then analyzed to measure an offset between a desired printhead position and the actual position of the printhead. The printheads are then mechanically moved to the desired position. The printheads may be moved with stepper motors, which in many instances cannot be simultaneously operated. Additionally, the alignment procedure may need to be repeated for a variety of reasons such as excessive measurement noise or backlash of the printhead motor screws. Throughout this process, the image substrate is fed through the device at full speed. Consequently, alignment procedures for printing systems which reduce the waste of media would be beneficial.